I was also wondering about the custom exhaust, but I will answer your other question... Yes there are other alternatives to renntech depending on what exact part you would like done... Since you mentioned renntech I am going to think that you are talking about their pulley + ecu upgrade... a decent alternative to that would be evosport... although not as reliable, but is a little cheaper and with a little more performance. As for the ecu tune... there are several who are able to do it... even some custom basement tuners probably...

Upgrades for this car come with a mercedes tax so don't expect anything cheap and if you find someone offering you the product for cheap decline it as it is likely incorrectly built or does not perform up to par...

HDDP mentioned an availability of camshafts from shrick?!? He will be better able to answer that... I plan to purchase these in time, but some recent things have set me back financially slightly and there are a couple of other mods I want to do before hand.

Here are a couple sites you can check for products for us:
http://www.evosport.com/
http://www.renntechmercedes.com/
http://www.kleemann.dk/site
http://www.startech.de/